Thin film temperature measurement using optical absorption edge wavelength

ABSTRACT

A technique for determining the temperature of a sample including a semiconductor film  20  having a measurable optical absorption edge deposited on a transparent substrate  22  of material having no measurable optical absorption edge, such as a GaN film  20  deposited on an Al2O3 substrate  22  for blue and white LEDs. The temperature is determined in realtime as the film  20  grows and increases in thickness. A spectra based on the diffusely scattered light from the film  20  is produced at each incremental thickness. A reference division is performed on each spectra to correct for equipment artifacts. The thickness of the film  20  and an optical absorption edge wavelength value are determined from the spectra. The temperature of the film  20  is determined as a function of the optical absorption edge wavelength and the thickness of the film  20  using the spectra, a thickness calibration table, and a temperature calibration table.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to Provisional Patent Application No.61/218,523, filed Jun. 19, 2009, the entire disclosures of which ishereby incorporated by reference and relied upon.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to methods for making precisenon-contact measurements of substrate and thin film temperature duringgrowth and processing of the thin film on the substrate.

2. Related Art

Advanced manufacturing processes involving depositing thin films onsubstrates often depend on the ability to monitor and control a propertyof the substrate, such as temperature, with high precision andrepeatability.

For many applications, precise temperature measurement during the growthof a thin film on a semiconductor wafer or substrate is critical to theultimate quality of the finished, coated wafer and in turn to theperformance of the opto-electronic devices constructed on the wafer.Variations in substrate temperature, including intra-wafer variations intemperature ultimately affect quality and composition of the layers ofmaterial deposited. During the deposition process, the substrate waferis heated from behind and rotated about a center axis. Typically, aresistance heater positioned in proximity to the wafer provides the heatsource for elevating the temperature of the wafer to a predeterminedvalue.

An example application illustrating the necessity of precise temperaturecontrol is the formation of semiconductor nanostructures. Semiconductornanostructures are becoming increasingly important for applications suchas “quantum dot” detectors, which require the self-assembled growth ofan array of very uniform sizes of nano-crystallites. This can only beaccomplished in a very narrow window of temperature. Temperatureuncertainties can result in spreading of the size distribution of thequantum dots, which is detrimental to the efficiency of the detector.

The growth of uniform quantum dots is an example of a thermallyactivated process in which the diffusion rates are exponential intemperature. Therefore, it is important to be able to measure, and haveprecise control over, the substrate temperature when growth orprocessing is performed.

Numerous methods have been disclosed for monitoring these temperatures.One simple, but largely ineffective approach has been the use ofconventional thermocouples placed in proximity to, or in direct contactwith the substrate during the thin film growth operation. Thismethodology is deficient in many respects, most notably, the slowresponse of typical thermocouples, the tendency of thermocouples (aswell as other objects within the deposition chamber) to become coatedwith the same material being deposited on the semiconductor wafer,thereby affecting the accuracy of the thermocouple, as well as the spotthermal distortion of the surface of the semiconductor wafer resultingfrom physical contact between the thermocouple and the substrate. In anyevent, the use of thermocouples near or in contact with the substrate islargely unacceptable during most processes because of the poor accuracyachieved.

Optical pyrometry methods have been developed to overcome theseshortcomings. Optical pyrometry uses the emitted thermal radiation,often referred to as “black body radiation,” to measure the sampletemperature. The principal difficulties with this method are thatsamples typically do not emit sufficient amounts of thermal radiationuntil they are above approximately 450° C., and semiconductor wafers arenot true black body radiators. Furthermore, during deposition asemiconductor wafer has an emissivity that varies significantly both intime and with wavelength. Hence, the use of pyrometric instruments islimited to high temperatures and the technique is known to be prone tomeasurement error.

In “A New Optical Temperature Measurement Technique for SemiconductorSubstrates in Molecular Beam Epitaxy,” Weilmeier et al. describe atechnique for measuring the diffuse reflectivity of a substrate having atextured back surface, and inferring the temperature of thesemiconductor from the band gap characteristics of the reflected light.The technique is based on a simple principle of solid state physics,namely the practically linear dependence of the interband opticalabsorption (Urbach) edge on temperature.

Briefly, a sudden onset of strong absorption occurs when the photonenergy, hv, exceeds the band gap energy E_(g). This is described by anabsorption coefficient:α(hv)=α_(g) exp [(hv−E _(g))/E ₀],

-   -   where α_(g) is the optical absorption coefficient at the band        gap energy. The absorption edge is characterized by E_(g) and        another parameter, E₀, which is the broadening of the edge        resulting from the Fermi-Dirac statistical distribution (the        broadening ˜k_(B)T at the temperatures of interest here). The        key quantity of interest, E_(g), is given by the Einstein model        in which the phonons are approximated to have a single        characteristic energy k_(B)θ_(E). The effect of phonon        excitations (i.e. thermal vibrations) is to reduce the band gap        according to:

${{E_{g}(T)} = {{E_{g}(0)} - {S_{g}k_{B}{\theta_{E}\left\lbrack \frac{1}{{\exp\left( {\theta_{E}/T} \right)} - 1} \right\rbrack}}}},$

Where S_(g) is a temperature-independent coupling constant and θ_(E) isthe Einstein temperature. In the case where T>>θ_(E), which iswell-obeyed for high-modulus materials like Si and GaAs, one canapproximate the temperature dependence of the band gap by the equation:E _(g)(T)=E _(g)(0)−S _(g) k _(B) T,showing that E_(g) is expected to decrease linearly with temperature Twith a slope determined by S_(g)k_(B). This is well obeyed in practiceand is the basis for band edge thermometry.

Variations on this methodology are taught by Johnson et al., in U.S.Pat. No. 5,388,909, and U.S. Pat. No. 5,568,978. These references teachthe utilization of the filtered output of a wide spectrum halogen lampwhich is passed through a mechanical chopper, then passed through alens, then through the window of the high vacuum chamber in which thesubstrate is located, and in which the thin film deposition process isongoing. Located within the chamber is a first mirror which directs theoutput of the source to the surface of the substrate. The substrate isbeing heated by a filament or a similar heater which raises thetemperature of the substrate to the optimum level required for effectiveoperation of the deposition process. A second mirror located within thechamber is positioned to reflect the non-specular (i.e., diffuse) lightreflected from the back surface of the substrate, said reflection beingdirected to another window in the chamber and thence through a lens to adetection system comprising a spectrometer. The wavelengths of theelements of the non-specular reflection are utilized to determine theband gap corresponding to a particular temperature. Johnson et al. teachthat the temperature is determined from the “knee” in the graph of thediffuse reflectance spectrum near the band gap.

While this prior art technique is in some ways effective, use of opticalfiber bundles, intra-chamber optics, mechanical light choppers andmechanically scanned spectrometers renders the methodology deficient inmany respects. The detected signal suffers from temporal degradation ofthe optics within the deposition chamber. The mechanical components areoverly susceptible to failure and the overall methodology of collectingthe signal is simply too slow for real-time measurement and controlapplications in the industrial production environment. In addition, thedescribed means of the prior art is subject to variations in accuracydependent upon the fluctuation, over time, of the output of the halogenlight source.

Specifically, this prior art relies on one or more optical elementswithin the deposition chamber to direct the incident light to the waferand to collect the diffusely reflected light. The presence of opticswithin the deposition chamber is problematic, since the material beingdeposited during the coating process tends to coat all of the contentsof the chamber, including the mirrors, lenses, etc. Over time thecoatings build up and significantly reduce the collection efficiency ofthe optics and can lead to erroneous temperature measurement.

More importantly, this prior art technique relies on a mechanical lightchopper and a mechanical scanning spectrometer for measurement of thelight signal. Not only do the mechanical components fail frequently withextended use, but it is well known that gears in scanning spectrometerswear, resulting in continual shifts in the wavelength calibration. Thisleads to perpetually increasing errors in temperature measurement unlessthe instrument is recalibrated frequently, which is a verytime-consuming process. In addition, it is well known that scanningspectrometers are quite slow, requiring anywhere from 1-5 seconds tocomplete a single scan. In most deposition systems the semiconductorwafers are rotating, typically at 10-30 RPM. In this case, a temperaturemeasurement that takes 1-5 seconds to complete is by default an averagetemperature and it is impossible to make any type of spatially resolvedmeasurement. If the process chamber has many wafers rotating on aplatter about a common axis, as is typical in a production depositionsystem, the slow response time of the prior art makes it impossible tomonitor multiple wafers.

Furthermore, the prior art utilizes a quartz halogen light source withno consideration of any type of output stabilization or intensitycontrol. Quartz halogen lamps are known to degrade rapidly over timeleading to fluctuations in the lamp output that result in measurementvariations and further system downtime for lamp replacement.

As alluded to above, control of the temperature or another propertyassociated with the process is best achieved through precise andreal-time monitoring of the substrate temperature or property. TheBandiT™ system from k-Space Associates, Inc., Dexter Mich., USA (kSA),assignee of the subject invention, has emerged as a premier,state-of-the-art method and apparatus for measuring semiconductorsubstrate temperature. The kSA BandiT is a non-contact, noninvasive,real-time, absolute wafer temperature sensor. The kSA BandiT systemprovides a viable solution for low-temperature wafer monitoring wherepyrometers cannot measure. The kSA BandiT system is also insensitive tochanging viewport transmission, stray light sources, and signalcontribution from substrate heaters. Diffusely scattered light from thewafer is detected to measure the optical absorption edge wavelength.From the optical absorption edge wavelength the temperature isaccurately determined. The kSA BandiT can run in two modes: 1)transmission mode, whereby the substrate heater is used as the lightsource and a single detector port is required, and 2) reflection mode,whereby the BandiT light source is mounted on one port, and the BandiTdetector unit is mounted on a 2nd, non-specular port. The kSA BandiT isavailable in two models covering the spectral range 380 nm-1700 nm. Dualspectrometer units are also available for applications requiring thefull spectral range. Typical sample materials measured and monitoredinclude GaAs, Si, SiC, InP, ZnSe, ZnTe, and GaN. The kSA BandiT systemis described in detail in US Publication No. 2005/0106876 and U.S.Publication No. 2009/0177432 the entire disclosures of which areincorporated here by reference.

Despite the widespread success of the kSA BandiT system, newapplications are emerging in which it is difficult to measure propertiesof the substrate, such as temperature, due to the non-semiconductorproperties of the substrate material. These non-semi-conductor materialsdo not have a measurable optical absorption edge and are typicallytransparent to all practical wavelengths of light. For example, blue andwhite light emitting diodes (LEDs) are manufactured by depositingGallium Nitride (GaN) onto a sapphire (Al₂O₃) or amporphous SiCsubstrate, which does not have a measurable optical absorption edge.Accordingly, the prior art temperature measuring techniques may not beviable in certain limited applications.

SUMMARY OF THE INVENTION

One aspect of the invention provides a method for determining atemperature of a semiconductor film having a measurable opticalabsorption edge deposited on a substrate having no measurable opticalabsorption edge. The method comprises the steps of providing thesubstrate material having no measurable optical absorption edge anddepositing the film of a semiconductor material having a measurableoptical absorption edge and a measurable thickness on the substrate. Themethod also includes interacting light with the film deposited on thesubstrate to produce diffusely scattered light. The method furtherincludes collecting the diffusely scattered light from the film andproducing a spectra indicating optical absorption of the film based onthe diffusely scattered light from the film. The method also includesdetermining a thickness of the film. The method further includesdetermining the optical absorption edge wavelength of the film, anddetermining the temperature of the film at the film thickness as afunction of the film thickness and the optical absorption edgewavelength.

Another aspect of the invention provides an apparatus for determiningthe optical absorption edge of a semiconductor film having a measurableoptical absorption edge deposited on a substrate having no measurableoptical absorption edge. The apparatus includes a detector forcollecting diffusely scattered light from the film and a spectrometerfor producing a spectra from the diffusely scattered light. Theapparatus further includes a software program for determining atemperature of the film at the film thickness as a function of the filmthickness and the optical absorption edge wavelength of the film usingspectra produced by the spectrometer.

Another aspect of the invention provides a system for determining anoptical absorption edge of a semiconductor film having a measurableoptical absorption edge deposited on a material having no measurableoptical absorption edge. The system includes a substrate of materialhaving no measurable optical absorption edge, a film of a semiconductormaterial having a measurable optical absorption edge and a measurablethickness deposited on the substrate, and a depositor for depositing thefilm on the substrate. The system also includes a light source forinteracting light with the film deposited on the substrate, a detectorfor collecting diffusely scattered light from the film, and aspectrometer for producing a spectra from the diffusely scattered light.The system further includes a software program for determining atemperature of the film at the film thickness as a function of the filmthickness and the optical absorption edge wavelength of the film usingspectra produced by the spectrometer.

The invention provides a real-time measurement of a property having adependence on film thickness, such as temperature of the film, when thefilm has a measurable optical absorption edge and the substrate has nomeasurable optical absorption edge. The invention takes advantage of thefact that the film is formed of a semiconductor material and thusprovides a measurable optical absorption edge wavelength. By accountingfor the dependence of the optical absorption edge wavelength of the filmon the film thickness, the invention can provide a precise, real-timemeasurement of the temperature of the sample.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages of the present invention will be readily appreciated,as the same becomes better understood by reference to the followingdetailed description when considered in connection with the accompanyingdrawings wherein:

FIG. 1 is a schematic view of an exemplary thin film deposition processincluding an optical absorption edge measurement system according to thesubject invention;

FIG. 2 is a schematic view of a second exemplary thin film depositionprocess including the optical absorption edge measurement systemaccording to the subject invention;

FIG. 3 is a fragmented perspective and cross sectional view of a filmincluding three layers deposited on a substrate;

FIG. 3A is an enlarged view of a section of the film and substrate ofFIG. 3;

FIG. 4 is a plot of intensity versus wavelength and includes a pluralityof spectra obtained as a single thin film of semiconductor materialdeposited on a substrate formed of non-semiconductor material at aconstant temperature;

FIG. 5 is a plot of the optical absorption edge wavelength versusthickness of a film at a constant temperature;

FIGS. 5A-C represent exemplary cross-sections of thin film and substrateat progressive times in a deposition process and corresponding generallyto the references 5A, 5B and 5C along the plot of FIG. 5, and furthershowing changes in diffusely scattered light as a function of filmthickness;

FIG. 6 is a simplified flow chart describing the process steps of theoptical absorption edge measurement method according to one embodimentof the subject invention;

FIG. 7 illustrates an example of the spectra produced during the spectraacquisition step;

FIG. 8 includes an example spectra and identifies oscillations employedto determine film thickness;

FIG. 9 is an enlarged view of the oscillations of FIG. 8;

FIG. 10 is a plot of thickness versus optical absorption edge wavelengthof a film;

FIG. 11 is dialog of a software program employing one embodiment of themethod of the subject invention illustrating thickness calculation;

FIG. 12 is dialog of the software program employing one embodiment ofthe method of the subject invention; and

FIG. 13 is a dialog of the software program including a thickness valueof a film.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the Figures, wherein like numerals indicate correspondingparts throughout the several views, an exemplary application of themethod, apparatus, and system for determining temperature of a sampleincluding a semiconductor film 20 having a measurable optical absorptionedge and a measurable thickness deposited on a substrate 22 having nomeasurable optical absorption edge is illustrated schematically in FIG.1 within the context of a film 20 deposition process. FIG. 1 shows asample within a deposition chamber 24. The sample includes a substrate22, such as a sapphire wafer, and a film 20 of a semiconductor material,such as GaN (Gallium Nitride) being deposited on the substrate 22, suchas sapphire, which is used as a component of blue and white lightemitting diodes (LEDs).

The system of FIG. 1 includes a light source 26 for interacting lightwith the sample, including the film 20 deposited on the substrate 22, toproduce diffusely scattered light. The light source 26 is typically aquartz halogen lamp, mounted outside the deposition chamber 24 whichdirects light toward the sample. The light provided by the light source26 is electromagnetic radiation of any wavelength, both visible and notvisible to the naked eye. A control unit 28 containing a lamp controllerunit 30 is connected to the light source 26 by a light source powercable. A computer 32, such as a laptop or standard central processingunit, employing a software program simultaneously monitors and operatesthe lamp controller unit 30 and other components of the system. Thecomputer 32 is connected to the control unit 28 by a USB cable 34.

In the application of FIG. 1, the system includes a heat source 36,which heats the substrate 22 and film 20 from behind as the substrate 22is rotated about a center axis and the film 20 is deposited on thesubstrate 22. Although not shown, the light source 26 and the heatsource 36 may be the same component. The temperature of the sample mustbe monitored and controlled as variations in temperature ultimatelyaffect quality and composition of the film 20 deposited on the substrate22. The system includes a temperature control 38, such as a PIDtemperature control 38, which is connected to the computer 32 and can bemanually operated by a user of the system.

As discussed in detail below, the light diffusely scattered from thesample is analyzed to determine the optical absorption edge wavelengthof the film, which is used to determine the temperature or otherproperties of the sample. The optical absorption edge can also bereferred to as the band edge or band gap. The system includes a detector40 for collecting diffusely scattered light from the film 20. Thedetector 40 is typically an Si-based detector 40. The detector 40includes a housing 42, which is also mounted outside the depositionchamber 24 proximate to a transparent view port at an angle that isnon-specular to the light source 26. The detector 40 includes anadjustable tilt mount 44 comprising a micrometer-actuated, single-axistilt mechanism built into the front of the detector 40 to assist inpointing the detector 40 at the sample within the chamber 24. Thedetector 40 also includes focusing optics 46 assisting in the collectionof the diffusely scattered light.

The system includes a spectrometer 48, such as a solid statespectrometer 48 or an array spectrometer 48, for producing a spectrafrom or based on the diffusely scattered light from the film andcollected by the detector 40. The optical absorption edge wavelength ofthe film is determined based on the spectra. The step of determining theoptical absorption edge wavelength of the film 20 based on the spectraincludes accounting for the semiconductor material and the thickness ofthe film 20, as discussed below.

The system includes an optical fiber unit 50, including a first opticalfiber 52 coupled to the spectrometer 48 and a second optical fiber 54running collinear to first optical fiber 52 and coupled to a visiblealignment laser 56 for aid in alignment of the detector 40. The opticalcomponents are optimized, using appropriate optical coatings, for eitherinfrared or visible operation depending on the characteristics of thesample being measured. The computer 32 is connected to the alignmentlaser 56 and the spectrometer 48 by the USB cable 34. The softwareprogram is employed to control the alignment laser 56 and spectrometer48.

The system can include a single apparatus for determining the opticalabsorption edge of the semiconductor film 20 having a measurable opticalabsorption edge deposited on the substrate 22 having no measurableoptical absorption edge. The single apparatus includes the detector 40for collecting diffusely scattered light from the film 20 deposited onthe substrate 22, the spectrometer 48 for producing the spectra from thediffusely scattered light, and the software program for determining anoptical absorption wavelength of the film 20 as a function of the filmthickness based on the spectra provided by the spectrometer 48. Thesoftware program can also determine the temperature of the film 20 as afunction of film thickness and optical absorption edge wavelength usingthe spectra produced by the spectrometer 48, which will be discussedfurther below.

The system typically includes a depositor or a means for depositing thefilm 20 on the substrate 22. The means for depositing the film 20 on thesubstrate 22 can include a chemical vapor deposition process such asmetalorganic vapor phase epitaxy (MOVPE), a molecular deposition processsuch as molecular beam epitaxy (MBE), or other thin-film depositionprocess such as sputtering.

FIG. 2 illustrates a second exemplary application of the method,apparatus, and system for determining an optical absorption edge of asemiconductor film 20 having a measurable optical absorption edgedeposited on a substrate 22 of material having no measurable opticalabsorption edge within the context of a film 20 deposition process. FIG.2 shows a substrate provided as a flexible steel sheet, which isprovided as a roll of material. The sheet is unrolled and transportedalong or across a conveyor as the film 20 is deposited on the substrate22. A plurality of clasps 58 maintain the substrate 22 in place as thefilm 20 is deposited.

As stated above, many new applications are emerging which requireprecise real-time monitoring of the thin film properties, such astemperature, during thin film deposition on a substrate 22 formed of amaterial that does not absorb light and thus has no measurable opticalabsorption edge. The substrate 22 is a non-semiconductor and istypically transparent to all practical wavelengths of light. One exampleof the substrate 22 material is sapphire (Al₂O₃), which is used to formblue and white LEDs, as discussed with regard to the application ofFIG. 1. Other examples of substrate 22 material having no measurableoptical absorption edge include SiO₂, glass, amorphous SiC, or metalssuch as thin rolled steel, Cu, Al, Mo, and Ta.

The film 20 deposited on the substrate 22 of these emerging applicationsis formed of a semiconductor material having a measurable opticalabsorption edge and a measurable thickness. The optical absorption edgeis measured according to methods discussed herein, and the thickness canbe measured by a variety of methods, as discussed below. Examples of thesemiconductor material of the film 20 include GaN and InGaN, which canbe deposited on the sapphire substrate and used to form the LED. Anotherexample of the semiconductor material of the film 20 is CdTe.Manufacturers of the LEDs formed of sapphire substrates 22 and GaN films20 typically require the substrate 22 to maintain a nearly constanttemperature, including a 1.0° C. or less deviation, when the film 20 isbeing deposited on the substrate 22. The temperature of the film 20deposited on the sapphire substrate 22 is determined from the opticalabsorption edge wavelength of the sample, as described below.

Monitoring the temperature of the sample including a film 20 formed of asemiconductor material deposited on a substrate 22 formed of anon-semiconductor material using the prior art kSA BandiT system andmethod, as described in US Publication No. 2005/0106876 and U.S.Publication No. 2009/0177432, typically produces inaccurate results.

The general dependence of the absorption of light by a semiconductormaterial is provided by Equation 1 below.I(d)/I(0)=1−exp(−αd)  Equation 1wherein d is the thickness of the film 20, I(d) is the intensity of thediffusely scattered light collected from the film 20 at the filmthickness (d), I(0) is the intensity of diffusely scattered lightcollected from the substrate 22 without the film 20, and α is theabsorption coefficient of the material of the film 20 at the band gapenergy of the material. The absorption coefficient of the material (α)accounts for the dependence of the optical absorption on the band gapenergy of the material, which is temperature-dependent. The absorptioncoefficient (α) is also referred to as α(hv) in the equation givenabove: α(hv)=α_(g) exp [(hv−E_(g))/E₀].

Equation 1 illustrates that the optical absorption of the film 20 isthickness-dependent and the behavior of the optical absorption isexponential. In applications wherein the substrate 22 has no measurableoptical absorption edge wavelength, light diffusely scatters from thesurfaces of the thin film 20, the interface between the film 20 and thethick substrate 22, and the surfaces of the substrate 22, likesubstrates formed of semiconductor materials. However, for substrates 22formed of semiconductor materials, the light is affected by thesubstrate 22, which has a large thickness, so the incremental changes inthe thickness have virtually no effect on the optical absorption edgewavelength. However, when the substrate 22 is formed of a materialhaving no measurable optical absorption edge wavelength, such as anon-semiconductor, the light is not affected by the substrate 20. Thesubstrate 22 is typically either transparent (e.g. glass or sapphire) orcompletely reflective (e.g. steel or other metal). Thus, the light isonly affected by the semiconductor film 20. Since the film 20 is thin,the incremental increases or changes in the film thickness have asignificant effect on the measured optical absorption edge wavelength ofthe film 20. An incremental change or increase in the film thickness istypically a 1.0 μm increase or decrease in thickness.

The prior art system and method do not adequately account for thethickness of the semiconductor material, which in this case is the thinfilm 20, rather than the thick substrate 22, when determining thetemperature of the sample. It has been determined that the inaccuratetemperature measurements obtained using the kSA BandiT system of theprior art are due to incremental changes in the thickness of the film 20being deposited on the substrate 22, and not accounting for the opticalabsorption edge wavelength dependence on thickness when determining thetemperature of the film 20.

In one embodiment, as shown in FIG. 3A, the film 20 includes threelayers 60, 62, 64 deposited on a substrate 22 of sapphire. The substrate22 has a thickness of about 600 μm. The base layer 60 deposited on thesubstrate 22 includes undoped GaN and includes a thickness of about 1.0μm to about 2.0 μm. The middle layer 62 deposited on the base layer 60is doped GaN and includes a thickness of about 0.5 μm to about 1.0 μm.The top layer 64 deposited on the middle layer 62 is InGaN and includesa thickness of about 0.2 μm to about 0.5 μm. The temperature of the toplayer 64 while it is being deposited on the substrate 22 and duringprocessing is especially crucial to the quality of the LED productbecause the top layer 64 produces the active layer of a multiple quantumwell. The temperature of the top layer 64 affects the color of the lightemitted from the LED product, and even slight temperature variations,such as +/−5.0° C. will yield noticeable differences in the light colorof the LED product. As alluded to above and shown in FIG. 3, the lightdiffusely scatters from the top and bottom surfaces of each of thelayers 60, 62, 64 of the film 20. The prior art kSA BandiT system andmethod, as described in US Publication No. 2005/0106876 and U.S.Publication No. 2009/0177432, which doesn't accurately account for thethickness of the film 20, produces inaccurate results for thetemperature of the sample in FIG. 3.

FIG. 4 explains how error and inaccurate results could occur when notaccurately accounting for the incremental change in the thickness of thefilm 20. FIG. 4 is a plot of intensity versus wavelength and includesseveral spectra obtained as a single thin film 20 of semiconductormaterial is deposited on the substrate 22 formed of non-semiconductormaterial at a constant temperature. Although the sample is maintained atthe constant temperature, the single sample produces spectra havingdifferent optical absorption edge wavelengths at each thickness. FIG. 5is a plot of the optical absorption edge wavelengths of the sample ofFIG. 4 as the deposited film 20 increases in thickness at a constanttemperature. The prior art method would provide results indicating adifferent temperature at each incremental change in the thickness of thefilm 20, which is not accurate.

The method, apparatus, and system of the present invention account forthe incremental changes in the thickness of the film 20 by determiningthe optical absorption edge wavelength of the film 20 as a function ofthe film thickness, which is then used to determine temperature of thefilm 20. The optical absorption edge wavelength and temperature aredetermined at a time during the manufacturing process when adjustmentscan be made to the film 20 to correct undesirable temperatures whichindicate undesirable properties.

As alluded to above, and shown in the flow chart of FIG. 6, the methodincludes depositing the film 20 of a semiconductor material having ameasurable optical absorption edge and a measurable thickness on thesubstrate 22, heating the substrate 22 and the film 20, and interactinglight signals with the film 20 deposited on the substrate 22 to producediffusely scattered light. The method next includes producing a spectraindicating optical absorption of the film 20 based on the diffuselyscattered light from the film 20. The method also includes determining athickness of the film 20, determining the optical absorption edgewavelength of the film 20, and determining the temperature of the filmat the film thickness as a function of the film thickness and theoptical absorption edge wavelength.

The first step includes performing a spectra acquisition to correctpotential errors due to equipment artifacts, such as a non-uniformresponse of the Si-based detector 40 used for 350 nm to 600 nmspectroscopy and non-uniform output light signals of Tungsten-Halogenlamps in the same wavelength range. These errors could prevent rawdiffuse reflectance light signals from yielding a measurable opticalabsorption edge at the correct wavelength position. When performing thespectra acquisition, it can be assumed the errors are steady-state.

The spectra acquisition first includes producing a reference spectrarepresenting the overall response of the system, i.e. the combination oflamp output signature and detector 40 response, which are bothwavelength dependent. The reference spectra, shown in FIG. 7, isproduced by interacting light with the substrate 22 without the film 20,for example bare sapphire, and collecting any of the diffusely scatteredlight in the detector 40. Next, the spectrometer 48 is used to generatethe reference spectra based on the diffusely scattered light collectedfrom interacting light with the substrate 22 alone. The spectraacquisition concludes by normalizing the reference spectra.

Each time a raw spectra is produced based on the diffusely scatteredlight from the film, the method includes normalizing the raw spectra anddividing the normalized raw spectra by the normalized reference spectrato produce a resultant spectra, as shown in FIG. 7. Dividing the rawspectra by the reference spectra is performed on every incoming rawspectra, and is necessary to determine an accurate film thickness, inaddition to enhancing the optical absorption edge signature. Theresultant spectra is normalized and used to determine the opticalabsorption edge wavelength. The resultant spectra provides a resolvableoptical absorption edge wavelength, which is used to determine thetemperature or another property of the film 20.

The spectra acquisition, including creating a normalized referencespectra, is performed each time a component of the system changes. Forexample, a view port of the detector 40 can become coated over time,which affects the collected light. The spectra acquisition can beperformed one time per run, one time per day, one time per week, or atother time intervals, as needed. Performing the spectra acquisition onetime per run will typically provide more accurate results than once perweek.

The spectra of the present method and system, including the referencespectra, raw spectra, and resultant spectra are typically produced byresolving the light signals from the substrate 22 into discretewavelength components of particular light intensity. The spectraindicates the optical absorption of the film 20 based on the diffuselyscattered light from the film 20. The spectra typically includes a plotof the wavelength versus intensity of the light, as shown in theFigures. However, the spectra can provide the optical absorptioninformation in another form, such as a table.

The resultant spectra are used to determine an optical absorption edgewavelength. As discussed in US Publication No. 2005/0106876 and U.S.Publication No. 2009/0177432, the optical absorption edge wavelength isthe abrupt increase in degree of absorption of electromagnetic radiationof a material at a particular wavelength. The optical absorption edgewavelength is dependent on the specific material, the temperature of thematerial, and the thickness of the material. The optical absorption edgewavelength can be identified from the spectra; it is the wavelength atwhich the intensity sharply transitions from very low (stronglyabsorbing) to very high (strongly transmitting). For example, theoptical absorption edge wavelength of the processed spectra of FIG. 7 isabout 425 nm. The optical absorption edge wavelength is used todetermine the temperature of the substrate 22.

The method includes producing a wavelength versus temperaturecalibration table (temperature calibration table) of a film 20 at asingle thickness. The temperature calibration table can also be providedto a user of the method, rather than produced by the user of the method.The temperature calibration table indicates the optical absorption edgewavelength versus temperature at a constant thickness of the film. Thetemperature calibration table provides subsequent temperaturemeasurements of the film based on the optical absorption edge wavelengthobtained from the spectra. However, unlike in the prior art system andmethod, the present system and method further includes determining thetemperature of the film 20 by accounting for the effect of the thicknessof the film 20 on the optical absorption edge wavelength, or thedependence of the optical absorption edge wavelength on film thickness,which will be discussed further below.

As stated above, the method and system of the present invention includesdetermining the optical absorption edge of the film 20 as a function ofthe film 20 thickness because the optical absorption edge wavelength ofthe film 20 is dependent on the thickness of the film 20. The filmthickness has an especially significant impact on the optical absorptionedge of thin films 20, and thus the determination of the temperature ofthe thin films 20, such as the top layer 64 of the sample of FIGS. 3 aand 3 b, including a thickness of about 0.2 μm to about 0.5 μm.

The thickness of the film 20 can be determined by a variety of methods.In one embodiment of the invention, the thickness of the film 20 isconveniently determined from the spectra produced by the light diffuselyscattered from the film 20 and used to determine the optical absorptionedge wavelength, discussed above. As shown in FIGS. 8 and 9, the spectraincludes oscillations below (to the right of) the optical absorptionedge region of the spectra. The oscillations are a result of thin filminterference, which is similar to interference rings observable on athin film of oil. A derivative analysis of the wavelength-dependentpeaks and valleys of the oscillations is employed to determine thethickness of the film 20. Equation 2 below can be employed to determinethe thickness of the film 20,

$\begin{matrix}{d = \frac{1}{2\left( {{n_{1}/\lambda_{1}} - {n_{2}/\lambda_{2}}} \right)}} & {{Equation}\mspace{14mu} 2}\end{matrix}$wherein d is the thickness of the film, λ₁ is the wavelength at a firstpeak of the oscillations and λ₂ is the is the wavelength at a secondpeak of the oscillations adjacent the first peak, or alternatively λ₁ isthe wavelength at a first valley of the oscillations and λ₂ is thewavelength at a second valley of the oscillations adjacent the firstvalley, n₁ is a predetermined index of refraction dependent on thematerial of semiconductor at λ₁, and n₂ is a predetermined index ofrefraction dependent on the material of semiconductor at λ₂. As shown inFIG. 9, the wavelengths used for λ₁ and λ₂ can be any two successivepeaks or any two successive valleys of the oscillations. Theoscillations and value obtained for thickness of the film 20 have anon-linear dependence on all layers 60, 62, 64 of the film 20. Thethickness of the film 20 can also be determined using other methods. Forexample, the thickness can be estimated based on previous measurementsof thickness as a function of deposition time or by laser-basedreflectivity systems such as the Rate Rat™ product available fromk-Space Associates, Inc., Dexter, Mich. USA.

As stated above, the step of determining the optical absorption edge ofthe film 20 as a function of the film 20 thickness includes accountingfor the dependence of the optical absorption of the film 20 on the filmthickness. The step of determining the optical absorption edge of thefilm 20 as a function of the film thickness can also include adjusting ameasured optical absorption edge wavelength value of the film 20obtained from the spectra due to the step of depositing the film 20 of asemiconductor material having a measurable optical absorption edge and ameasurable thickness on the substrate 22. The step of determining theoptical absorption edge of the film 20 as a function of the filmthickness can also include identifying the semiconductor material of thefilm 20 and adjusting a measured optical absorption edge wavelengthvalue determined from the spectra based on the semiconductor materialand the thickness of the film 20 to obtain an adjusted absorption edgewavelength.

The step of determining the optical absorption edge of the film 20 as afunction of the film thickness typically includes using a thicknesscalibration table. Each semiconductor material has a unique thicknesscalibration table. The thickness calibration table indicates opticalabsorption edge wavelength versus thickness at a constant temperature ofthe film.

The thickness calibration table can be acquired by growing a film 20 ofthe semiconductor material at a constant temperature and measuring theoptical absorption edge wavelength at each incremental increase inthickness to produce a spectra for each thickness. The thicknesscalibration table can also be prepared by depositing the film 20 on thesubstrate 22 at a constant temperature and measuring the opticalabsorption edge wavelength of the film 20 at the constant temperatureand a plurality of thicknesses. Preparing the thickness calibrationtable at a constant temperature also allows a user to determine thedependence of the optical absorption edge wavelength on the thickness.

The spectra acquisition is performed on each spectra, as describedabove. Next, from each of the spectra, a raw optical absorption edgewavelength value is determined for each thickness at the constanttemperature. An n^(th) order polynomial fit is performed on the rawoptical absorption edge wavelength values to produce the opticalabsorption edge wavelength versus thickness curve of FIG. 10 (solidline), where n is the order of the polynomial providing the best fit tothe data. This nth order polynomial dependence is used to create thethickness calibration table. The thickness calibration table is used asa thickness correction lookup up for subsequent temperaturemeasurements. The thickness calibration table illustrates the dependenceof the optical absorption edge wavelength on film thickness. As shown inthe embodiment of FIG. 10, the optical absorption edge wavelengthincreases as the film thickness increases. The thickness calibrationtable is produced for each unique semiconductor material, as differentmaterials produce different results. The thickness calibration table canalso be provided to a user of the method, rather than produced by theuser. However, for each unique material, only one thickness calibrationtable is needed to determine temperature of the film at variousthicknesses and temperatures. The method can include identifying thesemiconductor material of the film and providing the thicknesscalibration table and temperature calibration table for the identifiedsemiconductor material. The temperature of the film at a certainthickness is determined based on the spectra, the thickness calibrationtable, and the temperature calibration table.

Example 1

In one embodiment, determining the temperature of a GaN film 20 on asapphire substrate 22 includes first generating an optical absorptionedge wavelength versus thickness calibration table or curve (thicknesscalibration table) for a GaN film 20, as shown in FIG. 10. Next, themethod includes generating an optical absorption edge wavelength versustemperature calibration table (temperature calibration table) for a GaNfilm 20 at a single, predetermined thickness, for example 3.0 μm, usingthe kSA method disclosed in Publication No. 2005/0106876 and U.S.Publication No. 2009/0177432. The thickness calibration table at 3.0 μmand temperature calibration table is incorporated into the softwareprogram, which can be provided to a customer. The calibration tables areused to determine the temperature of a GaN film at other thicknesses.For example, the temperature of a film having a thickness of 4.0 μm isdetermined by producing a spectra at 4.0 μm. The optical absorption edgewavelength at 4.0 μm is determined from that spectra, for example 470nm. However, unlike the prior art method, that measured opticalabsorption edge wavelength of 470 nm is not used to determine thetemperature of the film. Rather, the present method includes referringto the thickness calibration table of FIG. 10 and determining thedifference between the optical absorption edge wavelength at 4.0 μm and3.0 μm, for example 3 nm, to obtain a wavelength difference. Next, thewavelength difference, 3 nm, is subtracted from the measured opticalabsorption edge wavelength of 470 nm to provide an adjusted or correctedoptical absorption edge wavelength of 467 nm. The corrected or adjustedoptical absorption edge wavelength of 467 nm is used to determine thetemperature of the film at 4.0 μm, using the temperature calibrationtable. The method also includes re-determining the temperature of thefilm 20 at incremental thicknesses to detect changes in temperature dueto the step of depositing the film on the substrate. The above steps canbe performed in real-time at each incremental increase in filmthickness, as the film is being deposited on a substrate. Since themonitoring and measuring is performed in real-time, any undesirabletemperature or other property of the material can be correctedimmediately to prevent harm in the quality of the product.

Example 2

The method of the present invention can be implemented in the softwareprogram installed in the computer 32 of the apparatus and systemdescribed above. The kSA BandiT system, discussed in US Publication No.2005/0106876 and U.S. Publication No. 2009/0177432 can be modified toinclude the method of the present invention.

The software program determines the film thickness using the spectra andEquation 2, discussed above. The step of determining the thickness ofthe film requires inputting several parameters, as shown in FIG. 11. Thefirst parameter is the number of slopes, which is defined as therequired number of slopes detected in succession that have at least theminimum slope defined by the user. The parameters also include minimumslope, which is defined as the minimum slope required to deem a peak orvalley as real and not noise; boxcar width, which is defined as theamount of boxcar smoothing of the raw spectra; minimum wavelength, whichis defined as the lowest wavelength of the spectra whereby thederivative analysis will be performed; and maximum wavelength, which isdefined as the maximum wavelength of the spectra whereby the derivativeanalysis will be performed. The dialog of FIG. 11 allows a user to setthe parameters for the peak and valley determination that issubsequently used in Equation 2.

The software program next determines the optical absorption edgewavelength as a function of the film thickness. The software programincludes a “band edge correction” feature, as shown in the dialog ofFIG. 12, which includes a thickness calibration table for thesemiconductor material of the film 20. The software program alsoincludes a temperature calibration table for the semiconductor of thefilm 20. The software program determines the optical absorption edgewavelength as a function of the film thickness using the calibrationtables, as described above. The measured optical absorption edgewavelength is adjusted or corrected based on the calculated filmthickness. The software program then employs the adjusted opticalabsorption edge wavelength to determine the temperature of the film 20,using the temperature calibration table and process of the prior art kSABandiT system, described in US Publication No. 2005/0106876 and U.S.Publication No. 2009/0177432.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings and may be practicedotherwise than as specifically described while within the scope of theappended claims. These antecedent recitations should be interpreted tocover any combination in which the inventive novelty exercises itsutility. The use of the word “said” in the apparatus claims refers to anantecedent that is a positive recitation meant to be included in thecoverage of the claims whereas the word “the” precedes a word not meantto be included in the coverage of the claims. In addition, the referencenumerals in the claims are merely for convenience and are not to be readin any way as limiting.

ELEMENT LIST Element Symbol Element Name 20 film 22 substrate 24 chamber26 light source 28 control unit 30 lamp controller unit 32 computer 34USB cable 36 heat source 38 temperature control 40 detector 42 housing44 adjustable tilt mount 46 focusing optics 48 spectrometer 50 opticalfiber unit 52 first optical fiber 54 second optical fiber 56 laser 58clasps 60 base layer 62 middle layer 64 top layer

What is claimed is:
 1. A method for determining temperature of asemiconductor film having a measurable optical absorption edge depositedon an optically transparent substrate material having no measurableoptical absorption edge, said method comprising the steps of: a)providing an optically transparent substrate of material having nomeasurable optical absorption edge, b) depositing a film of asemiconductor material having a measurable optical absorption edge and ameasurable thickness on the substrate, c) interacting light with thefilm deposited on the substrate to produce diffusely scattered light, d)collecting the diffusely scattered light from the film, e) producing aspectra indicating optical absorption of the film based on the diffuselyscattered light from the film, f) determining a thickness of the film,g) determining the optical absorption edge wavelength of the film basedon the spectra, and h) determining a temperature of the film at the filmthickness as a function of the film thickness and the optical absorptionedge wavelength.
 2. The method of claim 1 wherein said step h) includesaccounting for the dependence of the optical absorption of the film onthe film thickness.
 3. The method of claim 1 including re-determiningthe temperature of the film at incremental thicknesses to detect changesin temperature due to said step b).
 4. The method of claim 1 includingproviding a temperature calibration table indicating optical absorptionedge wavelength versus temperature at a constant thickness of the film,providing a thickness calibration table indicating optical absorptionedge wavelength versus thickness at a constant temperature of the film,and wherein said step h) includes determining a difference between anoptical absorption edge wavelength at the thickness determined by saidstep f) and the optical absorption edge wavelength at the thickness ofthe temperature calibration table using the thickness calibration tableto obtain a wavelength difference, subtracting the wavelength differencefrom the optical absorption edge wavelength determined by said step g)to provide an adjusted optical absorption wavelength value, and usingthe adjusted optical absorption wavelength value to determine thetemperature of the film at the thickness determined by said step f). 5.The method of claim 4 wherein said step of providing the temperaturecalibration table and said step of providing the thickness calibrationtable includes identifying the semiconductor material of the film andproviding the tables for the identified semiconductor material.
 6. Themethod of claim 1 wherein said step g) includes accounting for thesemiconductor material and the thickness of the film.
 7. The method ofclaim 1 wherein at least one of said step g) and said step h) includesemploying the following equation:I(d)/I(0)=1−exp(−αd) wherein d is the thickness of the film, I(O) is theintensity of diffusely scattered light collected from the substratewithout the film, a is the absorption coefficient of the semiconductor,and I(d) is the intensity of the diffusely scattered light collectedfrom the film at the film thickness (d).
 8. The method of claim 1including adjusting the thickness of the film and re-determining thetemperature of the film at the adjusted thickness.
 9. The method ofclaim 1 wherein said step h) includes determining the dependence of theoptical absorption edge wavelength of the film on the film thickness.10. The method of claim 9 wherein the step of determining the dependenceof the optical absorption edge wavelength on the thickness includespreparing an optical absorption edge versus thickness calibration tableat a constant temperature.
 11. The method of claim 10 wherein said stepof preparing a thickness calibration table includes depositing the filmon the substrate at a constant temperature and measuring the opticalabsorption edge wavelength of the film at the constant temperature and aplurality of thicknesses.
 12. The method of claim 1 includingnormalizing the spectra.
 13. The method of claim 12 wherein said step ofnormalizing the spectra includes: i) interacting light with thesubstrate prior to step b) to produce diffusely scattered light, j)collecting the diffusely scattered light from the substrate of step i),k) producing a reference spectra based on the diffusely scattered lightfrom the substrate of step j), l) normalizing the reference spectra ofstep k), m) dividing the normalized spectra of claim 12 by thenormalized reference spectra of step l) to produce a resultant spectra,and n) normalizing the resultant spectra of step m).
 14. The method ofclaim 13 wherein said step m) includes employing oscillations of thespectra of claim 12 below the optical absorption edge region of thespectra.
 15. The method of claim 14 including the step of employing aderivative analysis of the oscillations.
 16. The method of claim 14wherein said step m) includes employing the following equation:$d = \frac{1}{2\left( {{n_{1}/\lambda_{1}} - {n_{2}/\lambda_{2}}} \right)}$where d is the thickness of the film, λ₁ is the wavelength at a firstpeak of the oscillations and λ₂ is the wavelength at a second peak ofthe oscillations adjacent to the first peak or λ₁ is the wavelength at afirst valley of the oscillations and λ₂ is the wavelength at a secondvalley of the oscillations adjacent to the first valley, n₁ is apredetermined index of refraction dependent on the material ofsemiconductor at λ₁, and n₂ is a predetermined index of refractiondependent on the material of semiconductor at λ₂.
 17. The method ofclaim 1 wherein said step g) includes determining the optical absorptionedge wavelength as a function of the film thickness and band gap energyof the semiconductor material.
 18. The method of claim 1 wherein saidstep b) includes using at least one of a chemical vapor depositionprocess, a molecular deposition process and a sputtering process. 19.The method of claim 1 wherein said step b) includes transporting thesubstrate along a conveyor as the film is being deposited.
 20. Anapparatus for determining an optical absorption edge of a semiconductorfilm having a measurable optical absorption edge deposited on anoptically transparent material having no measurable optical absorptionedge comprising: a detector for collecting diffusely scattered lightfrom a film of a semiconductor material having a measurable opticalabsorption edge and a measurable thickness after it has been depositedon an optically transparent substrate of material having no measurableoptical absorption edge, a spectrometer for producing a spectra from thediffusely scattered light, and a software program for determining atemperature of the film as a function of film thickness and opticalabsorption edge wavelength of the film using spectra produced by thespectrometer.
 21. A system for determining an optical absorption edge ofa semiconductor film having a measurable optical absorption edgedeposited on an optically transparent material having no measurableoptical absorption edge comprising: a substrate of optically transparentmaterial having no measurable optical absorption edge, a film of asemiconductor material having a measurable optical absorption edge and ameasurable thickness deposited on said substrate, a means for depositingthe film on said substrate, a light source for interacting light withsaid film deposited on said substrate, a detector for collectingdiffusely scattered light from said film, a spectrometer for producing aspectra from the diffusely scattered light, and a software program fordetermining a temperature of the film as a function of film thicknessand optical absorption edge wavelength using spectra produced by thespectrometer.
 22. A system as set forth in claim 21 wherein saidsubstrate includes aluminum oxide (Al₂O₃) and said film includes galliumnitride (GaN).